Conventional vapour compression systems reject heat at the high pressure side by condensation of the refrigerant at sub critical pressure given by the saturation pressure at the given temperature. When using a refrigerant with low critical temperature, for instance CO2, the pressure at heat rejection will be supercritical if the temperature of the heat sink is high, for instance higher than the critical temperature of the refrigerant, in order to obtain efficient operation of the system. The cycle of operation will then be transcritical, for instance as known from WO 90/07683. Temperature and pressure on the high-pressure side will be independent variables contrary to conventional systems.
WO94/14016 and WO 97/27437 both describe a simple circuit for realising such a system, in basis comprising a compressor, a heat rejector, an expansion means and a heat absorber (evaporator) connected in a closed circuit. CO2 is the preferred refrigerant for both of them due to environmental concerns.
The above described transcritical cycle can also be used in multi-cooling systems, for instance in a super market system, in an industrial system or in a vending machine, which typically have a plural of evaporators and compressors in parallel. In contrast to conventional systems the pressure on the high pressure side, as also described above, can be controlled independently from temperature on the high pressure side. It exists an optimum or ideal pressure on the high pressure side, with a corresponding optimum, or maximum, system efficiency for a given operation condition, as described in WO 90/07683.
Each of the evaporators in the multi cooling system may have different and varying cooling demand, and hence requires an individual control of the refrigerant supply. Each evaporator is connected to an expansion means, which control the refrigerant supply to meet the varying cooling demands. The problem is to keep the optimum pressure on the high pressure side in the overall system, and at the same time serve all the demands of the evaporators. Optimum operation of such a system will need a special control strategy.
Normally, the individual refrigerant supply is controlled by separate valves which use the evaporator refrigerant superheat as input signal or control parameter. However, superheat makes the evaporators less efficient. Reduced superheat may give liquid pulsation of the evaporator and hence an instable temperature signal and possibly cycling of the valve control. It is neither possible to maintain, e.g. an optimum high pressure control, nor control a liquid level of a receiver at an intermediate pressure level, by using this control strategy. Charge variations of the active refrigerant introduced by this control strategy must be buffered and released at an intermediate pressure level or on the high pressure side if an optimal high pressure control is to be achieved. This makes an optimal control of the pressure on the high pressure side difficult due to very high design pressure for the components that would be required. A more robust and efficient design is therefore desirable.
A further problem for larger refrigeration plants, for instance in supermarket installations, is that the evaporator supply lines may become very long. In order to save cost, it may for high pressure refrigerants, such as CO2, be advantageous to switch to a lower pressure classification for the supply lines by reducing the supply refrigerant pressure. An optimized system design can ensure lower supply pressure.
WO 2004/057246 A1 describes a simple method for control of a refrigeration system that operates in transcritical mode, using for instance carbon dioxide as refrigerant. A simple and energy efficient control strategy is also needed when operating in sub-critical mode. Unlike conventional systems, only a limited part of the heat rejector will be used for condensation when using a refrigerant with a low critical temperature, for instance carbon dioxide. A new and simple method for optimum control at sub critical conditions is needed.
Evaporator coils for freezing applications (storage temperatures below 0° C.) need to be defrosted. The conventional way to perform defrosting is to supply heat by electric resistance heating rods mounted in the evaporator coil. The electric heating system increase evaporator production cost, increase running cost and increase coil size. By utilizing a proper system design, available process heat can be used for frost removal.